In recent years, many operations using a surgical microscope have been performed in order to make microscopic treatment possible in conjunction with the demand to minimize invasive surgery. Generally, surgical microscopes have an optical system built-in that provides a function for changing the observation magnification. For this reason, various treatments, such as suturing and so forth, can be performed under observation with the most appropriate magnification. For example, the most appropriate magnification may vary among a neurosurgical operation, the extraction of a tumor, the preventive treatment concerning a blood vessel which is developing abnormally, and so forth.
Further, a surgical procedure may not be performed on a flat surface, as it has been common for many years for surgery to be within a cavity. In a cavity, the light used to illuminate the surgical area is easily shielded by the entrance perimeter to the cavity when a surgical procedure is performed, especially when the cavity is deep. Accordingly, it is preferable that the optical axis of an illuminating beam be as closely aligned as possible to the optical axis of an observation beam for observing a surgical area in order for the illuminating beam to reach sufficiently deep into the cavity without being shielded.
As a result, various surgical microscopes have been set forth in the prior art that arrange the optical axis of an illuminating beam (hereinafter referred to as the illumination optical axis) so as to be nearly aligned with the optical axis of the observation beam (hereinafter referred to as the observation optical axis), or which arrange it to be matched with the direction of the observation optical axis.
As shown in FIG. 1 of Japanese Laid-Open Patent Application H8-257037, a conventional surgical microscope irradiates an illuminating beam onto a surgical area from a direction that is matched with the direction of the observation optical axis by arranging a beam splitter/combiner on the optical axis directly below the observation optical system. An illumination light flux is received from a direction that is normal to the direction of the observation optical system, and the beam splitter/combiner reflects the illuminating light beam so as to be co-linear with the optical axis of the observation optical system.
Other conventional surgical microscopes are made to irradiate an illuminating beam from two fixed directions to an observation object in the case where a surgical area is within a deep hole, as shown in FIG. 2 of Japanese Patent No. 3011950 and in FIG. 1 of Japanese Laid-Open Patent Application H10-73769. The illumination beam is divided into two illuminating beams in order to irradiate light into the deep hole, with the two illuminating beams being positioned symmetrically about the observation optical axis.
In addition, there are conventional surgical microscopes, as disclosed in FIG. 7 of Japanese Patent Publication H6-44101 and in FIG. 3 of Japanese Patent No. 2891923, which are made to illuminate a surgical area through an opening between left eye and right eye observation light fluxes, and an illumination optical system is constructed so as to direct the illuminating light to the surgical area from the opening.
In the surgical microscope disclosed in Japanese Laid-Open Patent Application H8-257037, less than one-fourth of the illumination of the beams emitted from the light source enter into the observation optical system, since only half of the illumination beam is reflected by the beam splitter/combiner toward the observation object and, of the illumination light that strikes the observation object and is reflected, only half is transmitted by the beam splitter/combiner and enters the observation optical system. Therefore, a surgeon must perform an operation while viewing either a dark image of the observation object or he must resort to using an expensive, high intensity light source which is capable of emitting a sufficient intensity of light to provide a bright image.
In the surgical microscopes disclosed in Japanese Patent No. 2891923 and in Japanese Laid Open Patent Application H6-44101, the left and right observation light fluxes, which correspond to the left and right observation optical systems, are on opposite sides of the illuminating light flux. Thus, even though the illuminating light flux reaches the bottom of a deep hole as a result of being aligned with the axis of the hole, the observation light fluxes become shielded by the entrance perimeter into the hole as the distance between the left and right light fluxes is large. Thus, the bottom of the hole will not be visible to an observer.
In the surgical microscopes disclosed in Japanese Patent No. 3011950 and Japanese Laid-Open Patent Application H10-73769, an inner wall of a deep hole can be illuminated brightly compared to the case of irradiating light from a single direction. However, observing bright images of a surgical area at the bottom of a deep hole has not yet been achieved because, in these conventional microscopes as well as other conventional microscopes, the angle between the illuminating light beams and the observation optical axis has not changed. Thus, there is a need to reduce the angle between the illuminating light beams and the observation optical axis in optical systems that efficiently use the available light so that bright images of a surgical area at the bottom of a deep hole can be observed.